Four-way open-center rotary control valves which use constant flow rate hydraulic power sources are commonly utilized for controlling vehicular power steering systems. Such systems typically employ a four-way open-center rotary control valve (hereinafter "rotary valve") having position feedback. Road feel is artificially induced by deflection of a torsion bar.
An earlier type of power steering system provided feedback partially proportional to actual steering effort. This power steering system featured a four-way open-center hydraulic reaction control spool valve (hereinafter "reaction valve"). However, such systems were rather complex to manufacture and were replaced by power steering systems which feature the simpler rotary valves mentioned above.
A rotary valve is a four-way open-center flow control valve which has circumferentially close fitting inner and outer valve members. The inner and outer valve members usually feature four sets each of pressure, first and second output, and return slots. These four sets of slots are equally spaced (at 90 degrees) around the interfacing circumferences of the inner and outer valve members. Differentially controlled output flows and/or pressures in the first and second output slots are obtained by rotationally displacing the inner valve member with respect to the outer valve member.
The open-center configuration of the rotary valve allows a nominally constant flow hydraulic fluid source to be utilized. In normal operation at other than small valve displacements, system supply pressure nominally approximates differential output pressure (hereinafter "output pressure"). This results in minimum system power consumption but results in wildly erratic system control characteristics wherein assist levels can vary by more than 40:1.
The advantages of the present invention can best be appreciated with reference to a thorough understanding of both the rotary valve and the reaction valve mentioned above. They will both be discussed in detail hereinbelow in a section entitled Detailed Description of the Existing Art.
In accordance with the present invention, a four-way open-center rotary control valve having internally generated hydraulic reaction torque (hereinafter "torque reaction valve") is provided. In preferred embodiments of the present invention, the hydraulic reaction torque is generated between inner and outer valve members which are formed with multiple control orifices having differing radii. The control orifices comprises input control orifices which meter fluid from a constant flow hydraulic fluid source into either of first and second output ports, and return control orifices which meter fluid returned therefrom to a tank.
The input control orifices are formed at smaller radii than the return control orifices. Thus, output pressure between the first and second output ports is additively applied to either side of each of a plurality of effectively enlarged ridge sections which form the return control orifices. The product of the output pressure, the summed areas of the enlarged ridge sections, and their effective radii generates the hydraulic reaction torque.
Output pressure is coupled to a utilization device, such as a power cylinder, via flow restrictors. The flow restrictors are controlled orifice devices which can have a nominally linear flow resistance characteristic. For this reason, values of differential pressure applied to the utilization device are less than the output pressure. The reduction of output pressure is nominally proportional to fluid flow rate through the utilization device. This results in a controlled damping ratio and stable operation of systems incorporating the flow restrictors of the present invention.
Improved performance can be obtained from a servo system comprising the torque reaction valve by introducing an orifice in parallel with a double acting utilization device also comprised within the servo system. Fluid flow rate through the orifice improves system damping and results in an improved control characteristic wherein over-sensitive response to small involuntary control inputs are precluded.